1. Field of the Invention
The invention relates generally to surface modified metallic particles that include organic molecules attached to the surface of metallic colloids, and more specifically to the use of such surface modified particles in analyte detection by surface-enhanced Raman spectroscopy.
2. Background Information
The ability to detect and identify trace quantities of analytes has become increasingly important in virtually every scientific discipline, ranging from part per billion analyses of pollutants in sub-surface water to analysis of cancer treatment drugs in blood serum. Raman spectroscopy is one analytical technique that provides rich optical-spectral information, and surface-enhanced Raman spectroscopy (SERS) has proven to be one of the most sensitive methods for performing quantitative and qualitative analyses. A Raman spectrum, similar to an infrared spectrum, consists of a wavelength distribution of bands corresponding to molecular vibrations specific to the sample being analyzed analyte). In the practice of Raman spectroscopy, the beam from a light source, generally a laser, is focused upon the sample to thereby generate inelastically scattered radiation, which is optically collected and directed into a wavelength-dispersive spectrometer in which a detector converts the energy of impinging photons to electrical signal intensity.
Among many analytical techniques that can be used for chemical structure analysis, Raman spectroscopy is attractive for its capability in providing rich structure information from a small optically-focused area or detection cavity. Compared to a fluorescent spectrum that normally has a single peak with half peak width of tens of nanometers (quantum dots) to hundreds of nanometers (fluorescent dyes), a Raman spectrum has multiple bonding-structure-related peaks with half peak width of as small as a few nanometers. Furthermore, surface enhanced Raman scattering (SERS) techniques make it possible to obtain a 106 to 1014 fold Raman signal enhancement, and may even allow for single molecule detection sensitivity. Such huge enhancement factors are attributed primarily to enhanced electromagnetic fields on curved surfaces of coinage metals. Such enhancement factors have also been observed on sharp edges and at the junctions between aggregates. Although the electromagnetic enhancement (EME) has been shown to be related to the roughness of metal surfaces or particle size when individual metal colloids are used, SERS is most effectively detected from aggregated colloids. It is known that chemical enhancement can also be obtained by placing molecules in a close proximity to the surface in certain orientations. Due to the rich spectral information and sensitivity, Raman signatures have been used as probe identifiers to detect a few attomoles of molecules when SERS method was used to burst the signals of specifically immobilized Raman label molecules, which in fact are the direct analytes of the SERS reaction. The method of attaching metal particles to Raman-label-coated metal particles to obtain SERS-active complexes has also been studied. A recent study demonstrated that SERS signal can be generated after attaching thiol containing dyes to gold particle followed silica coating.
Unfortunately, reliable methods for producing metallic colloids with consistent SERS performance have not yet been developed. In addition, there is a limited number of biomolecules (such as, for example, proteins) that adsorb to metallic surfaces to generate a SERS signal, and even for proteins that do adsorb, the signal intensity is low. Thus, a need exists for methods for producing metallic colloids with consistent SERS performance for detection of biomolecules such as proteins. In addition, there exists a need for methods for producing metallic colloids that are biomolecule specific.